The invention concerns an electrically rotatable adjusting shaft of a fully variable mechanical valve train of an internal combustion engine, said shaft comprising an adjusting cam.
The advantages of a throttle-free load regulation of Otto engines by means of fully variable inlet valve controls are known. By the omission of throttles, it is possible to exclude throttling losses that otherwise occur over a large range of load conditions of the internal combustion engine. This has a positive effect on fuel consumption and on the engine torque.
In variable mechanical valve trains, the stroke adjustment of the inlet gas exchange valves should be as spontaneous and exact as possible and should be effected at a high speed of adjustment. The adjusting mechanism is usually an adjusting shaft having locking curves or eccentrics.
Depending on the system used and the structural configuration, considerable moments of actuation are required for setting the desired valve stroke and the corresponding rotation of the adjusting shaft. These moments of actuation result from the reaction forces of the valve train that act on the adjusting shaft. For adjustment in a direction for obtaining a larger stroke, the adjusting shaft must be moved against the reaction forces of the valve train and, due to the oscillating movement of the gas exchange valves, this is accompanied by strongly pulsating torques.
To achieve an optimum operation of the valve train, a lash-free and extremely rigid support of the moments of the adjusting shaft is required. This support governs the positioning precision and the operation of a fully variable valve train as also the adjustability of an internal combustion engine equipped with such a system. The time for adjusting from a minimum to a maximum stroke should be less than 300 milliseconds.
The power requirement of the electric drive of the adjusting shaft should not put a too heavy load on the vehicle network. Therefore, small, high-speed electromotors combined with gearboxes having high transmission ratios are desirable.
One conceivable solution is to use worm drives. These, however, have a poor efficiency and are susceptible to wear that in its turn causes lash. In addition, worm drives have a limited range of transmission. It is also conceivable to use hydraulic adjusters similar to camshaft adjusters configured as vane-type adjusting devices or as coarse-thread adjusters. Their operation, however, depends to a large extent on the lubricating oil pressure which, in its turn, depends on the temperature of the lubricating oil and on the engine being actually in operation. Their adjusting dynamics and rigidity are low.
A further solution may be rotary drives but these have a low efficiency and a great amount of rotational lash.
It is an object of the invention to provide a compact actuator for the adjusting shaft of a fully variable mechanical valve train of an internal combustion engine, which actuator should have the highest possible rigidity and possess characteristics of low lash and low friction.
This and other objects and advantages of the invention will become obvious from the following detailed description.
The invention achieves the above objects by the fact that an actuator for rotating the adjusting shaft comprises an adjusting lever that is connected rotationally fast to the adjusting shaft, and a free end of the adjusting lever can be loaded by a cam plate that is driven by an electromotor. The connections between the adjusting lever and the adjusting shaft, between the adjusting shaft and the cam plate and between the cam plate and the electromotor are substantially free of lash and very rigid. This results in a high precision in the positioning of the rotating shaft and thus also in the adjustment of the inlet valve stroke.
In an advantageous embodiment of the invention, a roller preferably mounted in rolling bearings is arranged on the free end of the adjusting lever, which roller can roll on the cam plate, if necessary under spring bias, or in a gate of the cam plate. The rolling-bearing mounted roller assures low friction between the adjusting lever and the cam plate. When an alternating torque is applied to the adjusting shaft due to the kinematics of the valve train, the adjusting lever must be force-guided on the cam plate. This can be achieved through a gate or through an appropriate biasing of the adjusting lever by a spring.
Advantageously, the contour of the cam plate and of its gate, has a varying slope. Due to the configuration of the cam plate with a varying slope, a controlled variation of the transmission ratio can be realized as a function of its adjusting position. In this way, for example in the lower valve stroke region (the part-load region of the internal combustion engine), a high precision of adjustment and thus a fine load regulation can be achieved. On the other hand, the maximum torques applied to the adjusting shaft at full valve stroke can be distinctly reduced by a controlled reduction of the slope of the cam plate. By this, the torque of the electromotor as well, and thus the load on the vehicle network can be maintained at a low level and, in spite of this, a high speed of adjustment can be achieved.
Advantageously, the cam plate is driven by the electromotor through a gearbox. A gearbox in the drive of the cam plate permits the use of a high-speed electromotor with a relatively low torque. In the case of a direct drive, the electromotor would have to have a correspondingly high torque.
According to a further advantageous feature of the invention, the cam plate is made in one piece with an electromotor shaft or with a gearbox output shaft. As a result, it is possible to dispense with a coupling between the electromotor or the gearbox and the cam plate, so that a simple, compact actuator with high rigidity and freedom from lash is obtained.
Advantageously, the actuator comprises a fixed bearing that is configured as a deep groove ball bearing, an angular contact ball bearing or a four point bearing, and a movable bearing that is preferably configured as a needle roller bearing.
Because the electromotor is arranged parallel to the adjusting shaft and perpendicular to the plane of the adjusting lever, a particularly compact actuator is obtained.
Due to the fact that the transmission ratio between the electromotor and the adjusting shaft is determined by the transmission ratio of the gearbox, the slope of the contour of the cam plate and the length of the adjusting lever, a large range of transmission ratios can be realized in a single stage. The degrees of efficiency that can be achieved thereby are distinctly higher than with multi-stage rotary drives or with worm drives. Additionally, lash in the drive is only slight and the positioning precision is therefore high.
A further advantage of the invention is that the actuator can be installed in any longitudinal and any angular position on the adjusting shaft. In this way, the position of the actuator can be adapted to the installation and space conditions of the internal combustion engine.
Further features of the invention are disclosed in the following description and in the appended drawings which show a schematic representation of one example of embodiment of the invention.